1. Field of the Invention
The present invention relates generally to an apparatus for delivering a breathing gas to a patient and, more particularly, to a gas flow generator with manual standard leak adjustment.
2. Description of the Background Art
The sleep apnea syndrome affects some 1% to 5% of the general population and is due to upper airway obstruction during sleep. The direct consequences of sleep apnea are sleep fragmentation, partial cessation of ventilation and oxyhemoglobin desaturation. These in turn translate into daytime somnolence, cardiac arrhythmia, congestive heart failure and a variety of other health as well as cognitive dysfunctions. All of these have secondary social and behavioral effects which can result in increased patient morbidity as well as possible mortality if they are engaged in activities which require alertness (such as driving a car).
The causes of upper airway obstruction are varied but may include anatomical changes leading to a narrowing of the pathway, loss of muscle tone and/or increased weight of the structures. Age and obesity appear to be risk factors suggesting that an excess of soft tissue in the neck may provide sufficient pressure on internal structures to compromise the patency of the airway.
Treatment has involved a variety of surgical interventions including uvulopalatopharyngoplasty, gastric surgery for obesity, maxillo-facial reconstruction or even tracheostomy. All of these procedures have the risk of significant morbidity. A more benign treatment but one which requires some behavioral adjustment is that of nasal continuous positive airway pressure (nCPAP or just CPAP). In its simplest form, this treatment involves applying positive pressure to the airway using an airflow generator to force the passage to remain open. If used consistently during sleep, symptoms of sleep apnea can be successfully mitigated.
Some patients, however, are nonresponsive or noncompliant with CPAP treatment due to its continuous nature. This is especially true if the CPAP prescription pressure is relatively high. For these individuals a bilevel therapy is a more reasonable alternative. Pressure cycles from a high level during inhalation (IPAP) to a low level (EPAP) to facilitate exhalation while at the same time continuing to provide some nominal pressure support. This is also useful for individuals who have some form of compromised respiration such as a weakness of the diaphragm muscle due to disease or spinal injury where continuous pressure may be problematic.
All noninvasive flow generators whether they provide pressures at one level (CPAP), two levels (BiLevel/BiPAP) or multiple levels use a patient interface (typically a mask) which has some type of standard leak. The purpose of the standard leak is to allow carbon dioxide to leave the system and minimize rebreathing. Otherwise, significant tidal volumes would be required to clear the dead space of the hose and mask. The standard leak results from a purge hole (sometimes called an exhaust port or bleed hole) of a given size or numerous smaller holes. The size of these holes is determined by the flow capability of the generator and the desired properties of the mask.
In a home environment, flow generators may be used with minimal feedback to the user. In a hospital environment or sleep laboratory, however, it is desirable to know various measurable characteristics of the user""s breathing pattern. Typically these would include: the frequency of breathing, the ratio of the inspiration to expiration time (I:E ratio), the leak in excess of the standard purge hole leak, the tidal volume and the peak inhalation flow. The last three parameters in particular require knowledge of the purge hole leak for their correct calculation.
Various approaches have been used to calculate leak. For example, Estes et. al., (U.S. Pat. No. 5,901,704) discloses a method whereby total leak can be calculated and flow adjusted to compensate on a breath by breath basis. However, there is no discussion of how standard purge hole affects the calculation of tidal volume, excess leak or peak flow, nor do they discuss how multiple standard purge holes may be selected.
In another approach, a look-up table appropriate for a single type of mask has been used. For example, the KS 335 gas flow generator sold by Puritan Bennett contains a lookup table for leak in liters per minute which is indexed by the set pressure. The look-up table is based on a standard 4 mm hole in the mask. Masks with purge holes other than 4 mm would not produce correct readings on the device.
Other manufacturers have provided some type of automatic titration means or procedures such as those described by Brewer et. al (WO00037135). They describe a special xe2x80x9cmask fitxe2x80x9d mode wherein the device is set to a particular mask-fit test pressure, and the xe2x80x9caverage flowxe2x80x9d being the result of low pass filtering of the airflow is determined. This test pressure may be then stepped through the pressure range of interest and the results stored. This procedure must be repeated when masks are changed.
Others methods such as those disclosed by BerthonJones (WO9806449) estimate the nonlinear conductance of the mask orifice by dividing the average flow by the square root of the instantaneous measured pressure. This method, while versatile, is not accurate since purge hole leak must be estimated.
In accordance with one aspect of the present invention, an apparatus for delivering a breathing gas to a patient is provided which includes a blower that generates a flow of a breathing gas, a gas flow rate sensor positioned to sense the flow of breathing gas generated by the blower, a memory device containing a plurality of purge hole leak profiles corresponding to specific types of breathing masks, means for selecting one of the plurality of purge hole leak profiles from the memory device, and a microprocessor programmed to calculate at least one of excess leak, tidal volume, and peak flow using a flow rate measured by the gas flow rate sensor and the selected purge hole leak profile.
In accordance with another aspect of the present invention, a method for delivering a breathing gas to a user is provided which includes the steps of generating a flow of a breathing gas using a gas flow generator, measuring the flow rate of the breathing gas, selecting one of a plurality of purge hole leak profiles from a memory device, and calculating at least one of excess leak tidal volume, and peak flow using the measured flow rate and the selected purge hole leak profile.
In the preferred embodiment, the invention includes a display means, a selection means, and a storage means whereby different pressure versus leak curves may be selected for the purpose of calculating the standard leak appropriate to a given mask type. Accurate values of tidal volume, excess leak and peak flow may thereby be advantageously calculated. From the displayed excess leak value the fit of the mask may be corrected.